A7Al
A7Al Reference Phase Detector, Circuit Description
The A7Al Reference Phase Detector contains the frequency divider, phase detector, and integrating amplifier for the 100 MHz Reference phase-lock loop. Basically, 100 MHz from the A7A2 VCXO is divided by 10 and compared to the 10 MHz frequency reference by the phase detector. The error voltage from this comparison is fed back to the VCXO to keep its frequency locked to 10 times that of the frequency reference. The bandwidth of the reference phase-lock loop is 100 Hz; the 10 MHz derived from the 100 MHz VCXO must be within 100 Hz of the 10 MHz frequency reference for the loop to lock reliably.
Limiting Amplifier @
Ul amplifies and limits the amplitude of the reference signal from the rear panel FREQ REFERENCE input. For proper operation of the 100 MHz Reference phase-lock loop, this signal should be 5 MHz f25 Hz or 10 MHz f50 Hz at a power level of 0 dBm to +lO dBm. The internal A22A2 10 MHz Quartz Crystal Oscillator provides a stable, high spectral purity 10 MHz frequency reference at a power level of at least +7 dBm at the rear panel FREQ REFERENCE output. A short BNC jumper cable is normally connected between the FREQ REFERENCE input and output jacks to route the internal frequency reference signal to
the 100 MHz Reference phase-lock loop. The spectral purity of the FREQ REFERENCE input signal directly affects the instrument phase noise performance. The internal frequency reference remains off during instrument warmup (from a cold start) until it reaches thermal equilibrium.
Pulse Generator @
U2D is biased with feedback resistor RlO to further limit the 10 MHz to a well-shaped square wave and set the proper logic levels for digital buffer U2C. U2A and U2B generate narrow pulses, the width being the gate delay of U2A plus the delay from Rll and C5. When the output of U2C goes low, the output of U2B goes high after one gate delay (of U2B). After a delay due to Rll, C5 and U2A gate delay, the output of U2A goes high which causes U2B output to return low again, thus generating a narrow pulse.
Phase Lock Sampler @
The phase lock sampler performs the function of phase detector. The 10 MHz pulses from the buffer amplifier are applied to the primary of Tl which causes CR3 and CR4 to turn on for the duration of the pulses. This samples the divided by 10 VCXO frequency and stores this voltage on C36. When the loop is locked, the feedback due to the complete phase-lock loop forces this voltage to be nearly zero. When the loop is unlocked, this voltage may be zero or varying, depending on the reason for unlock.